1. Field of the Invention
The present invention relates to coated cutting tool insert particularly useful for turning of hardened steels and tool steels. Hard fine grained substrate in combination with a CVD-coating greatly improves the wear resistance. The coating is composed of one or more refractory layers of which at least one layer is a textured alpha-alumina (α-Al2O3).
2. Description of the Related Art
US 2009/017289 relates to a cutting tool insert for turning of steel at high cutting speeds. The cutting tool insert comprises a (006) textured α-Al2O3 coated by chemical vapour deposition on a cemented carbide substrate.
US 2009/016831 relates to a coated cutting tool insert, particularly useful for turning of heat resistant super alloys, utilizing a hard low Co-content submicron grain sized substrate in combination with a thin, very hard single (Ti,Si)N-layer grown using physical vapour deposition.
U.S. Pat. No. 7,201,956 relates to a cutting tool composed of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet, and a hard coating layer provided on the surface thereof; wherein the hard coating layer includes an aluminum oxide layer having an alpha crystal structure, with the highest peak in the inclination section of the (0001) plane of crystal grains relative to the normal of the surface.
Hardened steels cover a broad range of steels and properties and exist in many different conditions depending on purpose of use. As quenched, quenched and tempered, surface hardened (case hardened, nitrided, etc) are common conditions covering a hardness range up to 68 HRC. However, the purpose is to increase strength and wear resistance through obtaining different microstructures of the steels.
Steels suitable for hardening are medium to high carbon steels often with alloying additions of Cr, Ni, Mn and Mo. Depending on purpose, other alloying elements are added. Many of these alloying elements are carbide formers creating hard abrasive particles in the steels, which beside the high hardness, further decrease machinability and increase the wear on the cutting edge.
As strength and hardness are increased, the result is increased cutting forces and increased wear on the cutting edge during machining.
When cemented carbide cutting tools are used in the machining of hardened steels, the tool is worn by different mechanisms such as abrasive and chemical wear, chipping and fracturing of the cutting edge. For a coated tool insert normally having thin surface layers of wear resistant carbide, nitride, carbonitride and/or oxide compounds formed by various vapour deposition techniques, the coating contributes to increase the abrasive wear resistance, but it also acts as a thermal barrier for the diffusion of heat from the cutting surface into the underlying cemented carbide substrate. A high temperature within the edge region in combination with high cutting forces result in an increase of the creep deformation within the affected surface region of the substrate and the cutting edge deforms plastically. Inserts for machining of hardened steel must have good deformation resistance, wear resistance and toughness.